The present invention relates generally to methods for minimizing the frequency offset between satellite or wireless network components. More specifically, the present invention relates to method for measuring and reducing frequency offsets in distributed satellite and/or wireless networks. A corresponding system is also disclosed.
In a distributed satellite/wireless network based on Frequency/Time Division Multiple Access (FDMA, TDMA) technology, traffic terminals transmit data to one another in short bursts. Each terminal uses a local clock to generate carrier frequencies for transmission and reception of bursts. Due to a number of factors, the frequency at which a terminal transmits a signal is different than the frequency at which the signal is actually received at a destination terminal. This difference in frequency can be as high as 20 KHz. It will be noted, that this mismatch in frequency can have a large impact on the performance and reliability of the reception of a burst. In general, the larger the difference between the received frequency and the expected frequency, the larger is the probability of missing the burst or of detecting the burst but introducing bit errors in the process. Similar problems exist with respect to CDMA, i.e., Code Division Multiple Access, systems.
More specifically, the frequency mismatch between terminals is caused by the following factors:
(1) The up-converter equipment at the transmitter;
(2) The down-converter and up-converter equipment at the satellite;
(3) The down-converter equipment at the receiver; and
(4) The satellite Doppler caused by satellite motion.
It will be appreciated that most of these factors cause a time-varying change in frequency offset. For example, relatively rapid changes in frequency offset can occur due to temperature changes. Moreover, relatively slow changes in frequency offset can be attributed to equipment, e.g., terminal, aging.
Traditionally, this problem has been solved by designing satellite/wireless burst demodulators that are capable of handling large frequency offsets. It will be noted that this results in a more complex and expensive modem implementation. It should also be mentioned that this may dictate that transmission power levels be high so as to maximize burst recognition, which leads to further system complexities. For example, U.S. Pat. No. 5,619,525 to Wiedeman et al. discloses a method of operating a satellite communication system, which method provides adaptive closed loop power control. First, the ground station transmits an uplink reference signal with a first frequency to the satellite. The uplink reference signal experiences an attenuation between the ground station and the satellite due to, for example, a rain cell. The satellite then receives the reference signal and repeats the reference signal at a second frequency as a downlink reference signal that is transmitted from the satellite. The second frequency is less than the first frequency and is not significantly impaired or attenuated by the rain cell. The downlink reference signal is transmitted with a power that is a function of the power of the received uplink reference signal. Then, the downlink reference signal is received and used to determine the amount of attenuation that was experienced at least by the uplink reference signal between the ground station and the satellite. Thereafter, the transmitted power of the uplink reference signal is adjusted in accordance with the determined amount of attenuation so as to substantially compensate for the experienced attenuation. It would be preferable to avoid such complexities.
What is needed then is a method for minimizing frequency offsets in satellite networks and the like. U.S. Pat. No. 5,613,193 to Ishikawa et al., which is incorporated herein by reference, discloses a system and method for frequency offset compensation in a satellite mobile communication system. Frequency offset compensation is carried out by an Enhanced Automatic Frequency Control (EAFC) system in which a pilot (reference) signal is sent by a reference earth station physically separated from a land earth station. Using the pilot control signal, the land earth station measures the frequency offset in a signal received from a mobile earth station through a satellite due to frequency offset using a transponder in the satellite and a local oscillator in the mobile earth station, and the frequency offset due to Doppler shift by movement of the satellite. The land earth station then informs the mobile earth station of the measured frequency offset for controlling the local frequency for communication. Therefore, the Ishikawa et al. reference requires that each pair of terminals communicate and perform frequency measurements with respect to each other. Furthermore, not every terminal in the system proposed by the Ishikawa et al. reference measures frequency offset, i.e., between the mobile land stations.
Thus, what is needed is a method which permits a substantial reduction in the error between the actual received frequency and the frequency programmed at the demodulator. Advantageously, it would be desirable to have a method which permits a maximum error between the actual received frequency and the frequency programmed at the demodulator of several hundred hertz, with Ê100 Hz being considered a realistic maximum frequency error. It will be appreciated that this capability is critical where obtaining adequate performance at low signal-to-noise (SNR) levels is problematic using the conventional frequency error correction techniques discussed above.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a method for measuring and reducing frequency offset between components of a distributed system which overcomes the above-described deficiencies. The present invention was motivated by a desire to overcome the drawbacks and shortcomings of the presently available technology, and thereby fulfill this need in the art.
An object according to the present invention is to provide a method for measuring and reducing frequency offset in a distributed network, e.g., satellite network or wireless network, which results in a higher performance system. Advantageously, the higher performance system would be characterized by improved burst detection probability with a corresponding lower bit error ratio.
Another object according to the present invention is to provide a method for measuring and reducing frequency offset in a distributed network, e.g., satellite network or wireless network, which results in a lower cost implementation of, for example, the TDMA modem. According to one aspect of the present invention, instead of requiring frequency measurements between every pair of terminals, the inventive method implements an algorithm which requires measurements only between individual terminals and the reference terminal. According to another aspect of the present invention, these measurements are performed using reference and control bursts. It will be noted that this represents a significant departure from conventional method of frequency offset reduction, since the inventive method does not require that each pair of traffic terminals communicate with one another and perform frequency measurements with respect to each other.
Advantageously, the inventive method naturally allows reception of Aloha bursts, where a terminal can receive a burst from any transmitting terminal within a given time slot. Moreover, the method according to the present invention naturally allows transmission of multicast bursts, where multiple terminals can receive a burst from one or more transmitting terminals within a given time slot.
Yet another object according to the present invention is to provide a method for measuring and reducing frequency offset in a distributed network, e.g., satellite network or wireless network, which minimizes the number of parameters that each terminal has to maintain with respect to frequency management.
Still another object according to the present invention is to provide a method for measuring and reducing frequency offset in a distributed network, e.g., satellite network or wireless network, which minimizes the number of bursts dedicated for frequency offset management purposes.
These and other objects, features and advantages according to the present invention are provided by a method for measurement and reduction of frequency offsets in a communications network including a master reference terminal and a terminal exchanging reference and control bursts. Preferably, the method includes steps for adjusting demodulator frequency in the terminal responsive to a frequency error between a nominal frequency value and a respective reference burst received by the terminal, and adjusting modulator frequency at the terminal responsive to a frequency error between a nominal frequency value and a control burst generated by the terminal and transmitted to the master reference terminal.
These and other objects, features and advantages according to the present invention are provided by a method for measurement and reduction of frequency offsets in a communications network including a master reference terminal and N terminals exchanging reference and control bursts via a satellite, including steps for adjusting demodulator frequency in the Nth terminal responsive to a first frequency error between a first nominal frequency value and a respective reference burst received by the Nth terminal, determining a second frequency error between a second nominal frequency value and a control burst, the second frequency error being generated by the master reference terminal, and adjusting modulator frequency at the Nth terminal responsive to the second frequency error.
These and other objects, features and advantages according to the present invention are provided by a method for measurement and reduction of frequency offsets in a communications network including a master reference terminal, a secondary reference terminal, and N terminals exchanging reference and control bursts via a satellite. Advantageously, the method includes steps for adjusting demodulator frequency in the Nth terminal responsive to a first frequency error between a first nominal frequency value and a respective reference burst received by the Nth terminal, determining a second frequency error between a second nominal frequency value and a control burst, the second frequency error being generated by one of the master reference terminal and the secondary reference terminal, and adjusting modulator frequency at the Nth terminal responsive to the second frequency error.
These and other objects, features and advantages according to the present invention are provided by a communications network including a master reference terminal and a terminal exchanging reference and control bursts via a communications channel and having means for measuring and reducing frequency offsets in the terminal of the communications network, wherein a demodulator frequency of the terminal is determined based on a first frequency error between the reference burst and a nominal frequency value determined by the terminal and a modulator frequency of the terminal is established responsive to a second frequency error generated by the master reference terminal with respect to the control burst.
These and other objects, features and advantages of the invention are disclosed in or will be apparent from the following description of preferred embodiments.